The U.S. government levies a 35% tax rate on repatriated cash. That's a much higher rate than many companies currently pay, according to Oxfam. It said Apple's effective corporate tax rate was 25.9% between 2008 and 2014.

General Electric(GE) holds the second largest offshore cash stash, at $119 billion. Microsoft(MSFT), Pfizer(PFE) and IBM(IBM) round out the top five U.S. companies with the most offshore cash.

This issue has long flummoxed policymakers in Washington, some of whom are inclined to offer the companies a one-time tax break to entice them to bring the money home. But tax reform in the U.S. is slow at best, and the chance that lawmakers will tackle this issue in an election year is very slim.

Oxfam based its $1.4 trillion calculation on 2014 financial reports from 50 of the biggest companies in the U.S.

Oxfam said some of the cash may be re-invested in foreign subsidiaries, but still represents profits that escape U.S. taxation.